A porous cellulose body, which is also called aerogel, has low density and high specific surface area and is derived from natural products existing in a large quantity, and thus draws attention. A porous body consisting of organic matters generally has higher strength than that of a porous body consisting of inorganic materials at the same density in many oases. A porous body having high specific surface area is excellent in adsorbability, and can be a carbon-based porous body with carbonization by heating. In the middle of such technical trends, the present inventors have investigated utilization of cellulose nanofibers having a number average fiber diameter of 1 to 1000 nm.
The cellulose nanofiber in the specification, refers to (1) a fine cellulose nanofiber (cellulose fiber) or (2) a chemically-processed (modified) fine cellulose nanofiber having a number average fiber diameter of 1 to 1000 nm. The cellulose nanofiber of (1) is, for example, microfibrillated cellulose (hereinafter, abbreviated as MFC) that is fibrillated by shearing cellulose fibers under high pressure, or fine bacterial cellulose (hereinafter, abbreviated as BC) produced by a microorganism. The modified cellulose nanofiber of (2) is, for example, cellulose nanowhisker (hereinafter, abbreviated as CNW) obtained by treating natural cellulose with 40% or mere of concentrated sulfuric acid, or ultra-superfine and fine cellulose fibers having a uniform fiber diameter isolated as water dispersion with moderate chemical processing and slight machine treatment of microfibril, which is a minimum unit of a fiber constituting wood pulp, under ordinary temperature and ordinary pressure (for example, see Patent Literature 1).
A cellulose nanofiber is derived from a plant or an organism, and thus has a merit of low burden to the environment at the time of production and disposal compared to a nanofiber consisting of thermoplastic polymers derived from petroleum. Accordingly, it is expected that a porous body is formed using the cellulose nanofiber, and applied to various fields and uses such as a functional filter, an electronic device material, a recycled medical material, and further a carbon material.
However, a dried body of cellulose nanofibers obtained by drying a water dispersion of the cellulose nanofibers becomes a material having low fluid permeability and high density due to the cohesive force of the cellulose nanofibers working at the time or the drying. Particularly, a portion or the whole of the C6-position hydroxyl groups of the cellulose molecules on the fiber surface of the cellulose nanofibers having a number average fiber diameter of several nm described in Patent Literature 1, is substituted with a carboxyl group, which is more hydrophilic than a hydroxyl group. In addition, the surface free energy per unit mass generally increases as much as the fiber diameter is finer, and thus the cohesive force between the fibers, which stabilizes the surface at the time of the drying, increases. Accordingly, when the water dispersion of the cellulose nanofibers described in Patent Literature 1 is dried as it is, the water dispersion aggregates due to the hydrophilicity derived from the hydroxyl groups and the carboxyl groups of the cellulose and the strong surface tension possessed by water, and becomes a non-porous gas-barrier film (for example, see Patent Literature 2).
As a method of obtaining a porous cellulose body, disclosed is a method in which a water slurry containing minute cellulose fibers having an average fiber diameter of 2 to 1000 nm in 0.1 to 3.5 mass % of the solid content concentration is lyophilized, whereby to obtain a porous body having high surface area which has porosity and continuous foaming (for example, see Patent Literature 3).
As a method for preventing aggregation at the time of drying cellulose fine fibers (microfibrils), known are a method in which a water dispersion containing cellulose fine fibers is sprayed onto a cooled metal plate and rapidly frozen, and then sublimated to manufacture a porous body of the cellulose fine fibers, and a method in which the dispersion medium is substituted with ethanol and subsequently t-butyl alcohol, and then the dispersion is lyophilized whereby to manufacture a porous body of the cellulose fine fibers (for example, see Patent Literature 4).
As a method of obtaining a porous cellulose body (aerogel), known is a method in which water that is the dispersion medium in a physical gel of cellulose nanofibers is substituted with moisture ethanol, ethanol or t-butyl alcohol, and then the dispersion is lyophilized (for example, see Patent Literature 5).